In some conventional systems for wireless communication, a radio frequency (RF) signal is converted to an intermediate frequency (IF), and then from IF to a baseband signal. Generally, the RF signal may be heterodyned, or mixed with a local oscillator frequency that results in two signals that are centered about the sum of the two frequencies and the difference of the two frequencies. Usually the lower frequency, the difference of the two frequencies, may be chosen as the IF. Therefore, a radio that may be receiving several channels, such as an AM or FM radio, may tune to a particular station by changing the local oscillator frequency such that the IF remains constant. This is superheterodyning.
In superheterodyning, the local oscillator may need to be linked to the tuner because they both may need to vary with the carrier frequency. For example, if the IF is set to 40 MHz, in order to tune in a channel at 200 MHz, the local oscillator must be set to a frequency that will heterodyne the 200 MHz to the desired IF of 40 MHz. Therefore, the local oscillator must be set to 160 MHz, or alternatively to 240 MHz, so that the difference frequency will be exactly 40 MHz. When tuned to a different channel, for example, 210 MHz, the local oscillator must change to a frequency of 170 MHz, or 250 MHz, in order to generate an IF of 40 MHz. Accordingly, the local oscillator must be capable of varying the frequency over the same range as the tuner; in fact, they must vary the same amount. Therefore, the tuner and the local oscillator may be linked so they operate together.
There may be three main advantages to superheterodyning, depending on its application. First, the signal may be reduced from very high frequency sources where ordinary components may not work very well. Reducing the frequency may allow lower cost components to be used for a majority of the receiver circuitry. Second, the receiver and/or transmitter circuitry may be optimized and/or made more inexpensively for the IF. And third, superheterodyning may be used to improve signal isolation by arithmetic selectivity.
The ability to isolate signals, or reject unwanted ones, may be a function of the receive bandwidth. For example, a bandpass filter may be used to isolate the desired signal from adjacent signals, or other frequency sources that may interfere with the desired signal. If the performance of the bandpass filter isn't sufficient to accomplish this, the performance may be improved by superheterodyning. Frequently, the bandpass filter bandwidth may be some fraction of the received carrier frequency. For example, if the bandpass filter has a pass bandwidth of 2%, and receives the carrier at 200 MHz and a noise signal at 203 MHz, then signals within the range from 196 MHz to 204 MHz, 2% above and below the carrier frequency, may be passed. This range includes the noise signal 3 MHz above the carrier.
A superheterodyne receiver which may receive the same channel may bandpass filter the 40 MHz IF, and the bandpass filter may allow signals from 39.2 MHz to 40.8 MHz to pass. However, the noise signal, which may be 3 MHz above the desired signal may now be filtered out by the bandpass filter which may have the same characteristic of 2% pass bandwidth. The superheterodyne receiver may be said to be more selective.
However, in order to make superheterodyne receivers work, a reliable, stable source of local oscillator frequency is needed. Often, the local oscillator source may be buffered from the receiver and/or the transmitter circuitry. This may reduce unwanted noise to the local oscillator circuitry. The buffer may also serve to amplify the local oscillator signal to the receiver and or the transmitter circuitry.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.